Mirror Element Curvature Stability in EUV Lithography
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Solution Overview
Problem
Mirror elements in microlithographic projection exposure apparatuses experience unwanted changes in curvature and refractive power due to thermal expansions, leading to deterioration in optical properties, particularly in the EUV range where suitable light-transmissive refractive materials are lacking.
Innovation Solution
The design incorporates a substrate and layer stack with a reflection layer system, where the curvature is set based on a predetermined operating temperature, and includes a compensation layer or equalization layer to minimize thermal-induced changes in curvature, ensuring refractive power stability within a restricted temperature interval, thereby mitigating the bimetallic effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a layer stack with reflection layer system is applied onto a substrate to generate setpoint curvature, then the desired refractive power of the mirror element is achieved, but thermal expansion differences between the layer stack and substrate cause unwanted changes in curvature and refractive power during operation
Solution Approach 1:
The patent modifies the physical parameters of the substrate by introducing controlled defects (dislocations, inclusions, or curvature variations) to change its thermal expansion characteristics. This allows the substrate to compensate for the differential thermal expansion between the layer stack and substrate, maintaining curvature precision and refractive power stability across temperature changes during operation
Solution Approach 2:
The patent creates a composite structure by integrating a layer stack with reflection layer system onto a specially designed substrate with controlled defects. This composite construction combines the high-reflectivity properties of the layer stack with the thermally compensated properties of the defective substrate, achieving both manufacturing precision and operational reliability
2Ease of manufacture
If the substrate and layer stack have different thermal expansion coefficients, then the mirror element can be manufactured with initial curvature, but temperature changes during operation cause bimetallic effect leading to curvature variation
Solution Approach 1:
The patent changes the thermal expansion parameter of the substrate by introducing controlled defects. These defects modify the substrate's response to thermal stress, allowing it to maintain its curvature stability despite the bimetallic effect between the substrate and layer stack with different thermal expansion coefficients
Solution Approach 2:
The patent converts the harmful bimetallic effect into a beneficial compensation mechanism. By introducing controlled defects in the substrate, the differential thermal expansion between substrate and layer stack is transformed into a compensating mechanism that maintains curvature stability during temperature variations
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach maintains the mirror element's curvature and refractive power stability, reducing unwanted thermal effects by up to 10% over a 10 K temperature interval, thus preserving the optical system's performance.
Implementation Method 1
a curvature of the mirror element is generated on the basis of a setpoint curvature for a predetermined operating temperature by a non-vanishing bending force exerted by the layer stack
Implementation Method 2
the mirror elements are exposed to temperature changes (both during the commissioning and during the subsequent running operation of the respective optical system). In the case of differing thermal expansions of the layer stack on the one hand and the substrate on the other hand (i.e. as a consequence of the so-called bimetallic effect), this produces an unwanted change in the curvature or refractive power
Data Source
AI summary
A mirror element, in particular for a microlithographic projection exposure apparatus. According to one aspect, the mirror element includes a substrate (111, 112, 113, 114, 115, 211, 212, 213, 311a-311m, 411, 412, 413) and a layer stack (121, 122, 123, 124, 125, 221, 222, 223, 321a-321m, 421, 422, 423) on the substrate. The layer stack has at least one reflection layer system, wherein a curvature of the mirror element is generated on the basis of a setpoint curvature for a predetermined operating temperature by a non-vanishing bending force exerted by the layer stack, wherein the generated curvature varies by no more than 10% over a temperature interval (ΔT) of at least 10 K.